1. Field of the Invention
The present invention relates to an improved method and apparatus for a semiconductor gamma camera.
2. Prior Art
It is known within the art of medical diagnostic testing to utilize radioactive materials in determining a patient's condition. More specifically, the patient is administered a radioactive material which tends to concentrate itself with one or more particular patient organs or body parts. When this material concentrates itself within an organ it is possible, by utilizing radiation detection techniques, for a physician to observe and monitor the structure of a particular organ. From this information it is possible for the physician to diagnose the condition of that organ and therefore better treat the patient's condition.
One type of prior art radiation detection technique utilizes an "Anger" type gamma scintillation camera for determining the concentration and distribution of radioactive material as radiation is emitted from the patient's body. The Anger camera converts the radiation first into light energy and then into corresponding electrical impulses which are processed by imaging electronics to provide a display of radiation distribution. This technique, while of considerable utility, is limited in its ability to measure the energy of the radiation emitted from the patient and therefore the quality of the final image is less than optimum.
It has been subsequently proposed to bystep the conversion of radiation to visible light, as in the Anger camera, and convert radiation directly to electrical signals and therefore to improve spatial and energy resolution of the camera and of the displayed information, and provide for more efficient gamma radiation detection. In this newer technique the electromagnetic radiation (typically gamma ray radiation) impinges on intrinsic semiconductor material on which is imposed a high strength electric field. The gamma ray radiation impinging upon the structure of the semiconductor material produces holes and electrons within the material which, due to the influence of the electric field, are caused to separate and move toward opposite surfaces of the semiconductor material in accordance with their respective electrical charge polarities. After travel to opposed sides of the semiconductor material, the electron and holes are processed by decoding resistor circuits and attached imaging electronics to produce electrical signals indicating the location and energy content of the corresponding incident electromagnetic radiation. Prototype cameras embodying this principle (called "solid state" cameras) have been demonstrated and their improved performance capabilities confirmed.
In the typical fabrication of a solid state gamma camera, a high purity silicon or germanium crystal is utilized. This crystal forms the basis for a p-i-n diode with a large intrinsic region where charge carriers (holes and electrons) are created in response to electromagnetic radiation. The p-i-n diode is provided by doping opposed surfaces of a crystal with p and n type impurities or by forming metal rectifying contacts. The resulting p-i-n diode is reverse biased by an externally supplied electrical potential. Under these circumstances a large depletion volume can be maintained in the intrinsic region. When the electromagnetic radiation (typically gamma rays) impinges upon the semiconductor material, holes and electrons are formed.
If the spatial location of the incident radiation is to be determined as well as the energy content, the contacts are deposited upon the semiconductor material in a specific and well defined pattern. One technique utilized to provide information regarding the location of incoming radiation involves the deposition of bands of p and n doped material on the opposite surfaces of the semiconductor material. On one surface, only p doped material is utilized. This material is arranged in a series of bands or strips which form a parallel configuration of alternately doped and nondoped regions. On an opposed surface of the semiconductor material, the n doped bands are arranged parallel to each other but in a direction orthogonal to that of the p doped material. Each of the parallel bands of doped material is connected to a resistive network of discrete components.
To achieve this connection, each of the p and n doped materials must be electrically connected to this resistive network. Details of a location determining technique using a resistive network can be found in a publication entitled "A Practical Gamma Ray Camera System Using High Purity Germanium" published in the Feb. 1974 issue of IEEE Trans Nuc Sci and prepared by the Ohio State University Department of Nuclear Engineering under the auspices of a National Institutes of Health contract. That reference is incorporated by reference in the present patent application.
To provide more spatial information content or resolution, a greater number of parallel bands of n and p doped material are required. For each parallel band of p and n doped material, two electrical contacts must be made to the array of discrete component resistors. These resistor elements are attached to charge measuring circuitry in a manner illustrated in the referenced article. As the size of the semiconductor material increases, the number of electrical contacts on the doped bands increases as does the cost and complexity of the resistive networks necessary to obtain location information.
The semiconductor materials used to construct an improved resolution gamma camera are extremely pure and extreme care must be taken to insure the composition integrity of these semiconductors. Under these circumstances, it has been found that constructing the discrete component resistance networks and attaching them to the doped regions of the semiconductor gamma camera contributes significantly to the cost of such cameras.